Space-based Digital Audio Broadcast (DAB) systems provide a new type of service for distribution of CD-quality radio broadcast to mobile users such as automobiles and trucks. A salient property of Digital Audio Broadcast service is that it can be provided over a large geographic area when delivered from an overhead satellite.
Several digital Audio Broadcast systems are in use to provide service to the continental United States (CONUS). The service operated by XM Radio uses two geosynchronous-orbit spacecraft. Ideally, the DAB transmitter is located directly overhead of all mobile units to which service is provided, so as to prevent shadowing of the signal by adjacent structures or geographic features such as hills or mountains. With a location directly overhead, signal would be lost only when the vehicle passes through a tunnel or into a structure. A disadvantage of the use of geosynchronous spacecraft is that the required orbit is such that for northern latitude coverage, the elevation angle of the spacecraft as seen from potential users may be less than 60°, where the elevation angle is measured from the horizon. Thus, the XM Radio system has a potential for significant signal loss or shadowing, corresponding to loss of service, for at least some users or subscribers in northern latitudes. This can be corrected by the use of terrestrial repeaters, but such repeaters undesirably increase the cost and complexity of the system.
The DAB system operated by Sirius Radio, on the other hand, uses three spacecraft in highly elliptical inclined (HEO) “Tundra” orbits. This orbit has an inclination of 63.4° and eccentricity of 0.27, as it was originally developed to reduce orbital perturbations when providing service to Russia.
It has been determined that a Digital Audio Broadcast system should provide user-to-spacecraft elevation angles of no less than 70° over its service area in order to maximize user satisfaction. When applied to DAB service for Europe, the high orbit inclination of 64.3° results in poor elevation angle performance to users in central and southern Europe. Modified Molniya orbits have been proposed for DAB service to Europe, using eight spacecraft in twelve-hour orbits and an argument of perigee of about 240°.
Improved DAB systems are desired.